A chemical and mineralogical investigation of bloomery iron-making in the Bristol Channel orefield, U.K

• Main Author: Thomas, Gary
• Published: Cardiff University 2000
• Subjects: 669.141094298
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412525

Iron has been produced in the Bristol Channel region of the U. K. since the 3rd Century B. C., with evidence for the existence of this industry, in the form of slag, found abundantly across the area. The iron ores from the region are high purity goethite/haematite with extremely low trace element concentrations. The provenancing of ore fragments and smelting slags must rely on the relatively subtle chemical distinctions between the different sectors of the orefield. A series of discrimination diagrams are presented as an aid to provenancing ore fragments found at archaeological sites within the Bristol Channel orefield. Previous attempts to determine ore sources from slag chemistry have generally examined the major and trace element composition of the slags, but are complicated by the significant, but variable, concentrations of some of those elements in charcoal and furnace lining and by the mobility of some major elements during weathering. A technique for the graphical solution of the ore: furnace lining: fuel ash mixture in the melt and the yield of the reaction during bloomery iron smelting is presented. The technique focuses on the so-called immobile elements, particularly the rare earth elements, but employs a wide range of elements to show the important role that furnace lining plays in determining the chemistry of smelting slags. Data is presented from six iron-making sites across the region, which range from Iron Age to medieval in age. Slags from each site are described and mass balance calculations undertaken. The multi-element mass balance method is shown to have a number of functions; these include the modelling of ore compositions used in ancient smelting operations and assessingth e suitability of ore fragmentsf ound at archaeologicals ites for the production of analysed slag compositions.